An airborne moving target indicator (AMTI) radar is generally known and is the type of radar that has the capability to reject or cancel signals from fixed, or unwanted targets (non-movers), such as buildings, hills, etc. At the same time, such radars typically highlight or enhance the radar return signals from any moving targets (movers) such as aircraft, vechicles, or the like. One technique used in AMTI radar of the coherent type involves utilizing the doppler shift imparted to the reflected radar signals by a moving target as a part of a processing scheme to distinguish a mover from a non-mover. This doppler shift appears as a change in the phase of the received signals between consecutive illuminating radar pulses.
There are a number of problems which must be considered in the processing of radar returns where the AMTI radar is mounted in an aircraft. Because the aircraft is moving with respect to both the fixed and moving targets the radar returns from both target and clutter experience a frequency shift which can be corrected by known motion compensation techniques.
Synthetic-aperture radars are also generally known and such systems generally use a multiaperture antenna together with the movement of the platform on which the antenna is mounted as additional inputs into the processing of return signals in an AMTI radar. While this adds significantly to the complexity of the processing of the radar return signals, clutter cancellation to identify the movers can be significantly enhanced.
One well-known method of compensating for the effects of aircraft motion is known as displaced phase center technique and involves electronically displacing the antenna phase center along the flight path of the aircraft. Briefly, the technique involves the transmission and reception of radar returns by the antenna of the radar system having its phase center at a first known location. A second illuminating pulse is then transmitted and the return stored while the antenna has its phase center at a second known location. The phase centers of the first and second returns are separated by a precisely known distance related to the movement of the aircraft during the interpulse period and, knowing this information, the phase centers can electrically be changed to essentially coincide in time. At that point, the signals received by the multiaperture antenna from clutter, or stationary objects, will have properties suitable to cancellation leaving only the movers to be detected.
One technique for clutter cancellation is described in U.S. Pat. No. 4,093,950 issued Jun. 6, 1978 to T. apRhys for MOTION-COMPENSATION ARRANGEMENTS FOR MTI RADARS. The clutter suppression technique described in this patent is not limited to two pulses at a time but may be applied to a number of pulses. Phase and amplitude adjustments are also made to minimize the effects of antenna construction errors. The antenna subarray have phase centers which are separated by 2 VT. The sum and difference signals from each two adjacent subarray are taken to produce a sum channel and a difference channel for each group of subarrays. After adjustment of the difference channel signal in phase and amplitude, the latest return is added to a delayed return to produce a correction signal. That correction signal is then added to a delayed signal in the corresponding sum channel to provide a signal that is synchronized in time and phase with the most recent signal in the sum channel.
U.S. Pat. No. 3,735,400 issued May 22, 1973 to C. Sletten and F. S. Holt for AMTI RADAR CLUTTER CANCELING METHOD AND APPARATUS describes a three-aperture simultaneous mode clutter and canceller. This clutter cancellation technique is based on the premise that the return signals from stationary targets on the ground arrive at two antenna apertures with a unique and nearly linearly related phase delay as a function of doppler frequency if the antennas are displaced laterally along the aircraft flight path. Ground clutter cancellation can be acheived by a filter that separates the doppler spectrum into narrow channels and applies a given phase shift or delay to the returns in the narrow bandpass filter. Three channels of signal information from a three-aperture antenna are reduced to two clutter cancelled channel. This clutter cancellation technique utilizes only adjacent aperture channels from which the two channels are derived. Range integrations and phase comparisons are performed on each channel of information to provide target detection and angle measurement. One of the limiting characterisitcs of this processing technique is that the antennas must be in a line coincident with the velocity vector of the aircraft. Each of the antenna apertures are spaced apart from the adjacent aperture by a fraction of a wavelength, in this particular case one quarter of a wavelength. Another limitation is that the transmit aperture, this being one of the three receive apertures, is the same aperture as one of the receive apertures and so the transmit and receive antenna beamwidths are identical. This is significant because the resultant doppler spectra in each of the channels is not highly influenced by each beamwidth pattern of the individual receive apertures. Still another limitation of this approach is that the aircraft velocity must be sufficiently large to provide a clutter spectrum of 50 channels or more. Yet another limitation of this technique is that the bandpass filters are controlled by information from the aircraft navigation sensor which inherently has potential errors which should be considered. Also, this processing technique utilizes only three doppler filtering processes which necessarily provides less information compared to a system which incorporates a large number of filtering processes. And finally, although this disclosed technique has a means for compensating for antenna calibration errors, it does not include any compensation by the receive signals to correct for velocity and/or boresite errors.